The invention relates to the field of testing equipment for motor stators, and more particularly to equipment for testing stators for induction, stepping, and universal motors. Specifically, the invention relates to a method and apparatus for determining whether stator windings have been properly installed on an electric motor stator prior to final assembly of the motor.
Electric motors generally have a stationary part, known as a stator, and a rotating part, known as a rotor. Upon application of current to the motor, the rotor rotates relative to the stator, thereby converting electrical energy to mechanical energy. Typically, the stator has a generally cylindrical shape and annular cross-section with a longitudinally extending bore. The rotor is mounted on bearings and extends within the stator substantially the length of the bore. The stator is usually made of many thin laminations of iron or steel, each having the same cross-section as the overall stator, stacked one upon the other to build up the stator from end to end. Each of the laminations has a series of radially inwardly extending teeth with slots in between. When the laminations are stacked together, the teeth and slots cooperate to form slots that extend longitudinally the length of the stator. Conductors are laid within the longitudinally extending slots and wound or looped around the teeth to form field windings or coils. In operation, current is applied to external leads of the field windings to create magnetic fields within the stator. These magnetic fields interact with magnetic fields generated by the rotor to cause the rotor to turn.
Using modern manufacturing methods well known in the art, stators are commonly manufactured on production lines wherein the field windings are either wound directly on the stator by means of automated winding equipment, or are first wound by automated winding equipment and then fit into the stator slots in a process known as shed winding. In either process, the windings are intended to be wound in a specified direction, with a specified number of turns spanning a specified number of teeth, and with a predetermined absolute orientation within the stator. This ensures that the magnetic field produced by each winding when energized will have the correct position, polarity and strength to ensure proper operation of the motor when assembled.
However, the number of turns per winding, or the direction of the wound conductors, or the number of teeth spanned, or the absolute angular placement of the winding, can sometimes be incorrect due to problems with the manufacturing process. If a manufacturing defect is not discovered until after final assembly of the motor, then the motor must either be scrapped or undergo an expensive rebuild. Moreover, all of the production line operations performed on the motor after introduction of the fault will have gone to waste, increasing the overall cost of production. Also, waiting until after final assembly to discover a manufacturing defect can increase the difficulty in diagnosing the source of the problem and cause delay in correcting the problem.
Therefore, it is desirable to test motors and motor parts at various times throughout the manufacturing process in order to give immediate feedback as to the proper operation of the various manufacturing processes, to prevent wasted manufacturing steps on faulty motor parts, and catch repairable faulty motor parts when it is most economical to repair or replace them. For example, one method for testing electric motor stators on an assembly line comprises the steps of clamping the stator under test in a fixture, connecting the field windings to a source of power, inserting a test rotor into the stator, and applying power to the stator. If the rotor turns in the proper direction when the stator is energized, then it is assumed that the field windings are properly wound.
However, it has been discovered that stator windings have sometimes been wound in the wrong direction so as to produce a magnetic field with the wrong polarity, yet the test rotor has still turned in the proper direction. Also, it has been observed that the number of turns per winding can vary from the specified amount without producing a noticeable difference in the direction of rotation of the test rotor. This is undesirable in that improper turn ratios can lead to reduced motor efficiency and shortened motor life. In addition, many motors are optimized to run most efficiently when the windings have a specific angular placement with respect to the stator core. A simple direction test may not indicate improper winding placement.
Therefore, it is desirable to be able to test electric motor stators prior to final motor assembly to determine whether the field windings have been wound in the proper direction, with the proper number of turns per windings, with the windings spanning the proper number of teeth, and with the proper angular placement. Furthermore, it is desirable to be able to test electric motor stators in an automated fashion compatible with modern manufacturing processes. Moreover, it is desirable to be able to test electric motor stators in a way that permits accurate determination of stator faults and provides timely feedback as to possible sources of error in the manufacturing process.